Aggregate size distribution in flocculating dispersions

Abstract

In this work we demonstrate an instrument for measuring size distributions in colloidal dispersions. The technique involves measuring the intensity of the light scattered, at low or high angle, from individual particles as they pass through a laser illuminated volume. In an initially monodisperse flocculating dispersion, resolution of aggregates containing up to six singlet units may be obtained. Experiments are discussed which involve the diffusion-controlled (rapid) flocculation of polymer latex dispersions into deep potential energy minima. The results show that deviations from the aggregate size distributions, indicated by the classical Smoluchowski treatment, commence after about three half-lives of the process. The deviations are interpreted through a model which incorporates a correction to the collision probability based on modified diffusion coefficients and collision radii for the various aggregate species. The situation in which the depth of the potential energy minimum is reduced by surfactant addition is briefly examined. Here, deviations from the Smoluchowski distribution occur at a much lower level of aggregation and are more pronounced in that a much wider aggregate size distribution is observed.